This is the first systematic study of Sir Isaiah Berlin's political ideas to draw on all of his writings, including manuscripts, interviews, and correspondence. Claude Galipeau focuses on Berlin's defense of political liberalism, which is based on moral pluralism and an interpretation of the historical and cultural development of western civilization rather than on abstract arguments about rights. Galipeau brings out the uniqueness and force of this defense while acknowledging, in his discussion of Berlin's Zionism, the tensions and complexities inherent in it. Berlin's political thought offers a compelling justification of liberal institutions in the contemporary world and this study is an invaluable guide to the range and depth of his work.